Use malloc in rocksdb_transaction_get_snapshot (#8114)

Summary:
The snapshot structure returned by rocksdb_transaction_get_snapshot is
supposed to be freed by calling rocksdb_free(), so allocate using malloc
rather than new. Fixes https://github.com/facebook/rocksdb/issues/6112

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8114

Reviewed By: akankshamahajan15

Differential Revision: D27362923

Pulled By: anand1976

fbshipit-source-id: e93a8b1ffe26dafbe22529907f72b796ae971214

db/c.cc

@@ -4790,7 +4790,10 @@ void rocksdb_transaction_destroy(rocksdb_transaction_t* txn) {
 
 const rocksdb_snapshot_t* rocksdb_transaction_get_snapshot(
     rocksdb_transaction_t* txn) {
-  rocksdb_snapshot_t* result = new rocksdb_snapshot_t;
+  // This will be freed later on using free, so use malloc here to avoid a
+  // mismatch
+  rocksdb_snapshot_t* result =
+      (rocksdb_snapshot_t*)malloc(sizeof(rocksdb_snapshot_t));
   result->rep = txn->rep->GetSnapshot();
   return result;
 }